1. Field of the Invention
The present invention relates to a stratified ceramic body, oxygen sensor using the same and a fabrication method thereof.
2. Description of the Related Art
Generally, the air-fuel ratio of an engine for an automobile is measured by an oxygen sensor amidst exhaust gas components to detect the oxygen concentration thereof.
As an oxygen sensor, there conventionally exists for example an oxygen sensor of laminate type which integrates a solid electrolyte which becomes a main part of a sensing portion and a heater to heat the solid electrolyte.
This oxygen sensor has a pair of electrodes on both sides of the solid electrolyte. In order to protect the electrode which is positioned on the exhaust gas side, a porous protecting film is formed on a surface of the solid electrolyte to cover the electrode.
The foregoing protecting film is formed by plasma spraying of spinel powder onto the priorly baked surface of the solid electrolyte and the surface of the exhaust gas side electrode.
According to the foregoing method of the prior art, baking process of the solid electrolyte and formation process of the protecting film are performed separately, and so become a major factor for making the production process of the oxygen sensor complex.
For this reason, demand has increased for a simultaneous baking of a protecting film and a solid electrolyte in an oxygen sensor. However, the foregoing simultaneous baking necessitates coordination of the baking shrinkage rate of the protecting film and solid electrolyte, and realization is difficult.
Furthermore, it is essential that the protecting film formed on the exhaust gas side electrode of the oxygen sensor cause measured gas to pass and to reach the measured gas side electrode, and for this reason porosity is demanded. Meanwhile, it is demanded that the solid electrolyte be dense. Therefore, it is necessary that the protecting film and solid electrolyte not only allow simple simultaneous baking but also obtain a predetermined pore ratio in the protecting film after simultaneous baking, and simultaneous baking of the protecting film and solid electrolyte becomes even more difficult.
Accordingly, an oxygen sensor which employs ceramic starting material composed of secondary particles structured by cohesive bonding of primary particles as a protecting film of the electrode and which adjusts the baking shrinkage rate and grain size of the ceramic starting material by means of the grain size of the respective primary and secondary particles has been proposed conventionally (Japanese Patent Application Laid-open No. Hei 5-148039).
However, in this oxygen sensor as well, adjustment of the foregoing primary particles and secondary particles is still difficult.